An invisibility cloak in a diffusive light scattering medium

Ruiter, R.P. de (2015) An invisibility cloak in a diffusive light scattering medium.

Abstract:Hiding objects with an invisibility cloak appeals to the imagination and also has many practical applications, such as to hide unwanted objects, to improve stealth techniques and to extend the range of wireless devices. An invisibility cloak can make it seem as if an object is not there by guiding light around it. Similarly, cloaks can hide objects from magnetism or heat. In 2014 Schittny et al. presented an invisibility cloak based on the diffusion of light in a scattering medium. In such a medium light is scattered many times due to irregularities in the medium. Their cloak performed well for all wavelengths throughout the visible spectrum. In this research the cylindrical cloak of Schittny et al. is rebuilt and tested. Furthermore, methods to detect their cloak, i.e. breaking it, are discussed. The design of Schittny et al. relies on the theory of the diffusion of light. This description of the behavior of light in a diffuse medium is extensively discussed and the requirements for perfect cloaking are derived analytically. The cloak consist of a cylinder with a shell placed in a background medium. Perfect cloaking is obtained by matching the diffusivities of the cloaking shell and the background medium. This result is confirmed by finite element simulations using COMSOL Multiphysics. For a homogeneous illumination, the perfect cloaking condition results in a uniform intensity behind the cloak for the 2D case in the absence of absorption in the system and for perfect diffuse reflection at the cylinder. For the 3D simulation the resulting intensity was not completely uniform, but this is possibly caused by the used mesh size and the finite height of the system. In addition, physical experiments have been performed using a homemade cloak. The concentration of scatterers in the background medium was varied, leading to different diffusivities. The background medium was illuminated homogeneously on one side (inbound wall). The resulting intensity on the outbound wall of the container with the background medium was measured. A clear cloaking behavior was observed for diffusivities of the background that were close to the theoretical value for perfect cloaking. The intensity in the middle was still about 7% lower than the intensity of the background, probably caused by absorption of both the white paint on the cylinder as well as the shell itself. The quality of the cloak was determined by comparing the resulting intensity profile of the cloak with that of an obstacle and an obstacle with a transparent shell. The cloak showed a more uniform resulting intensity than the obstacle for the expected perfect cloaking condition. An obstacle with a transparent shell lies in between in terms of quality. Furthermore, it is shown that the ratio of diffusivities of the shell and the background medium of the system of Schittny et al. is wavelength dependent. This leads to 3% more intensity than the background behind the cloak than intended for red light. Also if there is a refractive index mismatch between the background medium, refraction and reflection occur. The photon density is then no longer the same as the required photon density for perfect cloaking. Both effects are promising to be exploited to detect the cloak.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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